This invention relates to an interface circuit for converting logic signal levels.
Generally, a power supply of about +5 V is connected to logic circuits constructed from bipolar transistors, for example a transistor-transistor-logic (hereinafter called a TTL) circuit. The high level of the TTL circuit output ranges from about 5 V to 2.4 V and the low level ranges from about 0.8 V to 0 V. Therefore, in the worst case for distinguishing the output levels, the high level of the TTL circuit output is 2.4 V and the low level is 0.8 V. On the other hand, a logic circuit constructed from MOSFETs (metal-oxide-semiconductor field-effect transistor) (hereinafter called a MOS-logic-circuit) is generally driven by a power supply, for example, of about +10 V, different from that of a TTL circuit. In this MOS-logic-circuit, the boundary between a high level and a low level is about +5 V. So in the worst case, the high level of the TTL almost coincides with the low level of the MOS-logic-circuit and the signal of the TTL circuit cannot drive the MOS-logic-circuit directly. Therefore it is necessary to insert an interface circuit to raise the output levels of the TTL circuit up to a sufficiently high level to drive the MOS-logic-circuit correctly. And, when using the same power supply, it is necessary to insert an interface circuit in order to drive all MOSFETs correctly. It is difficult to manufacture many circuits which have a uniform circuit threshold voltage for the reason that it is difficult to manufacture many MOSFETs having a uniform threshold voltage. So, in the case of manufacturing the MOS-logic circuit driven by the TTL signal, the levels of the TTL-output-signal must be raised by connecting the output terminal of the TTL circuit through a resistor to the power supply of the MOS-logic-circuit. In this way, the voltage level of the connecting point of the resistor is at the power supply potential or ground potential.
But, generally this method is expensive because the resistor is provided outside the ICs (integrated circuits) which are used in the MOS-logic-circuit and TTL circuit. There is a way to construct a resistor on an IC together with the MOS-logic-circuit. But, in case the TTL-signal drives many MOS-logic-circuits, the output signal levels of the TTL circuit often change according to the number of resistors which are connected in parallel with the output terminal of the TTL. As a result, the circuit often causes miss action and it is difficult to drive the MOS-logic-circuit properly.